Liquid crystal displays (LCDs) are used in a wide variety of computers and consumer devices such as TVs. A back-lit LCD is an array of pixels in which each pixel acts as a shutter that either passes or blocks light from a light source that is located behind the pixel. Color displays are implemented by equipping the pixels with color filters such that each pixel transmits or blocks light of a particular color. The intensity of the light from each pixel is set by the time the pixel is in the transmissive state.
The display is typically illuminated by a white light source that provides a uniform intensity of light across the back surface of the display. Illumination sources based on fluorescent lights are particularly attractive because of their high light output per watt-hour of power consumed. However, such sources require high driving voltages which makes them less attractive for battery operated devices.
LCD displays for use in handheld devices such as cellular telephones and PDAs must be very thin, as the display represents a significant fraction of the thickness of the device. Light sources based on fluorescent lights are poorly adapted for such thin displays. Back-lit illumination systems for LCD arrays typically utilize some form of light box or light pipe behind the LCD array. Light is injected into this light box at the periphery of the light box. The surface of the light box opposite to the surface that is adjacent to the LCD array has some form of scattering covering that scatters the light so that the back surface of the LCD is uniformly illuminated. The preferred light source is a linear source that generates white light. The linear source is aligned with the edge of the light pipe. To provide efficient coupling of the light into the light pipe, the linear source must have a thickness that is significantly smaller than that of the light pipe.
The thickness of the light source is limited by the thickness of the light box. The thickness of the display is particularly important in displays used for laptop computers and handheld devices such as photodetector arrays and cellular telephones, as the display thickness limits the overall thickness of the device. Some of these portable devices require light boxes that are less than 10 mm thick. As the thickness of the light box is reduced, solutions based on fluorescent lights become more difficult to implement and still maintain a high power conversion efficiency.
As a result, there has been considerable interest in utilizing light sources based on LEDs in such applications. LEDs have similar electrical efficiency and long lifetimes. In addition, the driving voltages needed are compatible with the battery power available on most portable devices. In addition, the size of the light emitter in an LED source is much less than 1 mm. Hence, a linear source constructed from a plurality of LEDs can provide high coupling efficiency even with the thin light pipes discussed above.
A display is typically constructed from an LCD panel that is backlit via a white light source. The panel consists of an array of pixels in which each pixel includes an LCD light gate that either transmits or blocks light from the backlight. Each pixel corresponds to the light of a particular color that is to appear at a particular point in the generated image. Hence, each pixel includes a bandpass filter that selects light of a particular color from the white light generated by the backlight. Typically, three colors are used, red, blue, and green. Hence, an image that is to have N image points requires 3N pixels.
The intensity of light at each pixel as perceived by a person viewing the screen is determined by the time period in which the shutter is open rather than by altering the intensity of the light passing through the shutter. Consider a motion picture that is being displayed on the display. The picture consists of a series of frames that are projected in sequence. The time period that each frame is displayed is so short that the eye can only measure the average light intensity from each pixel over the time period. Hence, a pixel that is open for twice the time that a neighboring pixel appears to be twice as bright, even though the actual intensity of light leaving each pixel is the same during the time periods in which the pixels are open.
An LED light source for illuminating such a display is typically constructed from three colors of LEDs. The relative intensities of the LEDs are adjusted by adjusting the drive current through each LED in light sources for LCD panels that utilize the intensity scheme described above. The light source is typically constructed by alternating the red, blue, and green LEDs along a line that is parallel to an edge of the light pipe to simulate a linear white light source.
Unfortunately, LEDs suffer from aging problems. As the LED ages, the drive current through the LED must be increased to compensate for the aging of the LED. Since the aging effects are different for different color LEDs, the perceived color of the display will shift with age unless the drive currents are altered. In one class of light sources, the intensity of light in each of the color bands is measured by a corresponding set of photodiodes. The drive conditions are then adjusted to maintain the output of the photodiodes at a set of predetermined values corresponding to the desired perceived color for the light source. This approach assumes that all of the LEDs of a particular type age at the same rate and that the LEDs of a given type are driven in series. However, even with this approximation, the additional circuitry and detectors associated with the monitoring operation are a significant factor in the cost of the display.